Automatic ping when certain patterns occur in the diff?

[RalfJung]

The #[rustc_allow_const_fn_unstable] attribute should only be added with wg-const-eval approval. However, it hard to ensure that all reviewers know about this, and it is easy for a reviewer to miss this even if they are generally aware of this policy.

So could we get some bot help for that? If a PR adds new occurrences of this attribute, can we get an automatic ping from rustbot? Ideally, PRs removing the attribute wouldn't trigger the ping. So only lines with a + in the diff should be considered. Also, it might be worth restricting this to the part of the diff that is inside the library/ folder.